An active matrix liquid crystal display device such as a liquid crystal display uses a thin film transistor (hereunder referred to as TFT) as a switching device and comprises: a transparent pixel electrode; a wiring part including a gate wiring and a source/drain wiring; a TFT substrate having a semiconductor layer composed of amorphous silicon (a-Si) or polysilicon (p-Si); a counter substrate being disposed oppositely apart from the TFT substrate at a prescribed interval and having a common electrode; and a liquid crystal layer embedded between the TFT substrate and the counter substrate.
In a TFT substrate, pure Al or an Al alloy such as Al—Nd is generally used as a wiring material for a gate wiring or a source/drain wiring because it has a low electric resistivity and can be patterned easily. Meanwhile, a problem such as RC delay of wiring (a phenomenon of delay of electric signals transferring in wiring) becomes obvious in accordance with the upsizing of a liquid crystal display and needs for a wiring material having a lower resistance are increasing. Consequently, pure Cu or a Cu alloy such as Cu—Ni (hereunder collectively referred to as a Cu alloy) having a lower electric resistivity than an Al alloy has been attracting attention.
As described in Patent Literatures 1 to 6, a barrier metal layer comprising a high melting point metal such as Mo, Cr, Ti, or W is generally formed between a Cu alloy wiring film (Cu alloy film) and a semiconductor layer of a TFT. The reason is that, when a Cu alloy wiring film is brought into direct contact with a semiconductor layer of a TFT without interposing a barrier metal layer, Cu in the Cu alloy wiring film diffuses into the semiconductor layer due to thermal history in a succeeding process (for example, a process for forming an insulating layer on the TFT or a thermal process such as a sintering or annealing process) and the TFT properties deteriorate. More specifically, electric current (off-state current at the time of switch-off or on-state current at the time of switch-on) flowing in the TFT or the like is adversely affected, the increase of the off-state current or the decrease of the on-state current is caused, and also a switching speed (responsiveness to an electric signal of switch-on) lowers. Further, it sometimes happens that the contact resistance between the Cu alloy wiring film and the semiconductor layer increases.
A barrier metal layer is effective in inhibiting interdiffusion between Cu and Si at the interface between a Cu alloy film and a semiconductor layer as stated above but, in order to form a barrier metal layer, a film forming device for barrier metal forming is required separately in addition to a film forming device for Cu alloy wiring film forming. More specifically, a film forming device equipped additionally with a film forming chamber for barrier metal layer forming (typically, a cluster tool in which plural film forming chambers are connected to a transfer chamber) has to be used and that causes a production cost to increase and productivity to lower. Further, since an etching rate at a working process such as wet etching using a chemical liquid or the like is different between a metal used as a barrier metal layer and a Cu alloy, it comes to be very difficult to control working dimensions in the lateral direction at a working process. As a result, the forming of a barrier metal layer causes: processes to be complicated; a production cost to increase; and productivity to lower, from the viewpoints of not only film forming but also working.
In the above case, explanations have been made by exemplifying a liquid crystal display device as a representative example of a display device. The aforementioned problem caused by interdiffusion between Cu and Si at the interface between a Cu alloy film and a semiconductor layer is seen not only in a display device however but also in a semiconductor device such as an LSI or an FET. In the production of an LSI that is a representative example of a semiconductor device for example, a Cu alloy film is formed after a barrier metal layer composed of Cr, Mo, or TaN is formed over a semiconductor layer or an insulator layer in order to prevent Cu atoms from dispersing from the Cu alloy film into the semiconductor layer or the insulator layer and the simplification processes and the reduction of costs are desired also in the field of a semiconductor device.
For that reason, provision of a technology that can avoid the problem caused by interdiffusion between Cu and Si occurring in a display device or a semiconductor device even when a barrier metal layer is not formed unlike a conventional case is desired.
In view of the above situation, a direct contact technology that can omit the formation of a barrier metal layer and can bring an Al alloy wiring used for a source/drain electrode or the like into direct contact with a semiconductor layer, which is a technology of using not a Cu alloy but pure Al or an Al alloy as a wiring material, is proposed in Patent Literatures 7 to 9. In the Patent Literatures, Patent Literature 9 is disclosed by the present applicants and discloses a wiring structure in which a material comprising a nitrogen containing layer and an Al alloy film is used and N (nitrogen) in the nitrogen containing layer bonds with Si in a semiconductor layer. It is estimated that the nitrogen containing layer functions as a barrier layer for preventing interdiffusion between Al and Si and it is verified that excellent TFT properties can be obtained even when a barrier metal layer composed of Mo or the like is not formed unlike a conventional case. Further, another advantage is that, since the nitrogen containing layer can be formed easily by applying nitriding treatment such as plasma nitridation after a semiconductor layer is formed and before an Al alloy film is formed, a specific film forming device for barrier metal forming is not required.